Tetrafunctional initiator

ABSTRACT

The melt of polyvinyl aromatic polymers comprising from 10 to 45 weight % of star branched polymer prepared using a combination of thermal and tetra functional peroxide initiation has an improved melt strength permitting better foam formation for extrusion foam blown with conventional blowing agents and inert gases including CO 2  and an improved tensile strength for oriented polystyrene (OPS) articles, e.g. blown film or extruded sheet.

This application is a continuation-in-part of U.S. Ser. No. 09/678,910filed on Oct. 4, 2000 which is a division of U.S. Ser. No. 09/553,593filed on Apr. 20, 2000 which issued into U.S. Pat. No. 6,166,099 on Dec.26, 2000.

FIELD OF THE INVENTION

The present invention relates to polymeric foams and oriented articles,e.g. blown film or extruded sheet and a polymer composition used in thefoams and oriented articles. More particularly the present inventionrelates to foams and oriented articles prepared from a polymercomposition comprising a vinyl aromatic polymer that comprise from 10 to45 weight % of a star branched vinyl aromatic polymer.

BACKGROUND OF THE INVENTION

Monovinyl and vinyl aromatic-based resins, such as for example,styrene-based resins, i.e. polystyrene-based resins, are well known andwidely employed for producing foams and oriented articles for use infood packaging, toys, small appliances, compact disc and audio/videocassette casings. Processes used to manufacture such articles involveextrusion, fast injection molding, blow molding, and thermoformingapplications.

In the manufacture of extrusion foam there are competing factors tobalance. One needs to consider the viscosity or melt flow rate of thepolymer as it impacts on the extruder output and the melt strength ofthe polymer, and particularly of the foaming mass as it leaves theextruder as this impacts on the bubble stability or the foam stability.If one makes a very low viscosity polymer it will flow through theextruder easily. However a low viscosity polymer tends to have a lowmelt strength and the resulting foam tends to have a lower stability.Accordingly, there is a tendency for foams of low viscosity to collapseupon extrusion or shortly after leaving the extruder.

It has been known for some time that the melt strength of a polymer maybe improved by lightly cross-linking the polymer. The paper “SomeEffects of Crosslinking Upon the Foaming Behavior of Heat PlastifiedPolystyrene”, L. C. Rubens Journal of Cellular Plastics, April 1965,311-320 discloses that polystyrene, containing small amounts (about 0.03weight %) of divinyl benzene, may be foamed with CO₂ and the polymer hasgood foam stability and good foam volume. This technology is also thesubject matter of U.S. Pat. Nos. 2,848,427 and 2,848,428 issued Aug. 19,1958 to Louis C. Rubens assigned to The Dow Chemical Company. Thetechnology comprised forming a cross-linked polystyrene polymer thenimpregnating it in solid state with CO₂ then releasing the pressure andletting the polymer expand. This technology was not strongly relevant toextrusion foam techniques.

The cross linking technology was further applied in U.S. Pat. No.3,960,784 issued Jun. 1, 1976 to Louis C. Rubens assigned to The DowChemical Company. This patent teaches concurrent impregnation of apolymer with a blowing agent and a cross-linking agent. The polystyreneis prepared at temperatures from about 60° C. to 120° C., preferablyfrom about 70° C. to 100° C. (Column 3 lines 25-26). These temperatureranges are indicative of suspension polymerization and concurrent orpost polymerization impregnation with the blowing agent and crosslinking agent (see Example 3) although the polymer could be molded intothin sheets for the impregnation step. This reference does not teachextrusion foam.

While divinyl benzene is useful in suspension polymerization it tends toproduce gels in bulk or solution polymerization. In a bulk or solutionpolymerization the use of tetra functional initiators significantlyreduces gels. Typically no or very low levels (e.g. less than 0.5 weight%, more generally less than 0.1 weight %) of gels (i.e. insolublepolymer in typical solvents) are desired.

With the introduction of the Montreal protocol on reducing the use ofCFC's and HCFC's and regulations regarding the permissible discharge ofvolatile organic compounds (VOC'S), there was increased pressure on thepolymer foam industry to move to other blowing agents such as CO₂ or N₂.Representative of this type of art is Monsanto's Australian Patent529339 allowed Mar. 17, 1983. The patent teaches the formation of a foamby extruding polystyrene and injecting CO₂ into the extruder.Interestingly there is no mention of cross linking agents or branchedpolystyrene in the patent. U.S. Pat. No. 5,250,577 issued Oct. 5, 1993to Gary C. Welsh is similar as it pertains to extrusion foamingpolystyrene in an extrusion process using CO₂ as the sole blowing agent.Again there is no reference in U.S. Pat. No. 5,250,577 to the use ofcross-linking agents.

At about this time U.S. Pat. No. 5,266,602 was issued to Walter et al.and assigned to BASF. This patent teaches foaming a branchedpolystyrene. The foaming agent is conventional (e.g. C₄₋₆ alkanes). Thepolymer is prepared in the presence of a peroxide initiator other than abenzoyl compound and a chain transfer agent such as a mercaptan (e.g.t-dodecyl mercaptan) and a “branching agent”. The branching agentcontains a second unsaturation as a point for the polymer to branch.Suitable agents include divinyl benzene, butadiene and isoprene. Thesetypes of branching agents would not produce the star branched polymersreferred to herein. The actual polymerization process appears to be asuspension process. Additionally there is no reference in the disclosureto blowing the polystyrene with anything other than conventional alkaneblowing agents.

U.S. Pat. No. 5,576,094 was issued on Nov. 19, 1996 to Callens et al.and assigned to BASF. This patent teaches extruding slab foamedpolystyrene blown with CO₂ or a mixture of CO₂ and C₁₋₆ alcohols orethers of C₁₋₄ alkyl alkoxy compounds. The polystyrene is a branchedpolystyrene preferably having at least 50%, more preferably 60% of thepolymer being a star branched styrene butadiene block polymer. Thepolymer has a VICAT softening temperature not greater than 100° C. Thisteaches against the subject matter of the present invention.Additionally the polymer has a melt index MVI 200/5 of at least 5 mL/10minutes.

U.S. Pat. No. 5,830,924 was issued on Nov. 3, 1998 to Suh et al. andassigned to The Dow Chemical Company. This patent claims a process forextruding a closed cell foam using CO₂ or a mixture of CO₂, conventionalalkane blowing agents and a polystyrene in which from 50 to 100 weight %of the polystyrene is star branched (i.e. branched). This teaches awayfrom the subject matter of the present invention that requires adifferent type of polymer and lower weight % of star branched vinylaromatic polymer.

U.S. Pat. No. 5,760,149 was issued on Jun. 2, 1998 to Sanchez et al.This patent discloses tetra functional (monoperoxycarbonate) compoundsthat are useful as initiators for olefin monomers including styrene. Thepatent also teaches a process for polymerizing polystyrene. However,there is no teaching in the patent of foaming the resulting polymerusing extrusion techniques.

Oriented film or sheet may also be made from styrenic polymers. Examplesof oriented articles, e.g. films, sheets, or tubes, are disclosed inU.S. Pat. Nos. 4,386,125; 5,322,664; 5,756,577; and 6,107,411.

U.S. Pat. No. 4,386,125 was issued on May 31, 1983 to Shiraki et al. andassigned to Asahi Kasei Kogyo Kabushika Kaisha. This patent discloses atransparent film, sheet, or tube of a block copolymer or a blockcopolymer composition having an excellent low-temperature shrinkage ofnot less than 15% in terms of a heat shrinkage factor at 80 ° C. in atleast one direction and good mechanical properties. The block copolymerhas a melt flow of 0.001 through 70 grams/10 min. and comprises anaromatic vinyl hydrocarbon polymer block having a number averagemolecular weight of 10,000 through 70,000 and a polymer block composedmainly of a conjugated diene, and a residual group of a coupling agentor a polyfunctional initiator such as an organo polylithium compound.

U.S. Pat. No. 5,322,644 discloses a method and apparatus for making aclear single layer polystyrene non-foam film for use as a label oncontainers. A blend of general purpose polystyrene and styrene-butadieneor styrene butyl acrylate is extruded from an annular extruder dieorifice to form a frustoconical tube which is stretched before coolingair is applied to form a clear film that has machine directionorientation and cross direction orientation that can be used as ashrinkable label on containers. During the extrusion stage, thepolystyrene has flow rates of about 8-10 (grams/10 min. condition G) andVICAT softening temperatures of about 220 to 225° F.

U.S. Pat. No. 5,756,577 was issued on May 26, 1998 to Villarreal et al.and assigned to Group Cydsa, S.A. de C.V. This patent claims a heatshrinkable thermoplastic film or sheet comprising a block copolymer ofstyrene-butadiene, wherein the amount of polymerized butadiene units inthe copolymer constitutes from about 1 to about 50 weight % of thecomposition. The film or sheet has a tensile strength of about 372kg/cm² in the machine direction and about 255 kg/cm² in the transversedirection, and a shrinking value at 130° C. of about 44% for the machinedirection and about 0% for the transverse direction.

U.S. Pat. No. 6,107,411 was issued on Aug. 22, 2000 to Toya et al. andassigned to Denki Kagaku Kogyo Kabushiki Kaisha. This patent disclosed ablock copolymer consisting essentially of a vinyl aromatic hydrocarbonand a conjugated diene, which is excellent in transparency, stiffness,impact resistance, and spontaneous shrinkage resistance; a compositioncomprising such a block copolymer, and heat shrinkable films prepared byorienting them. The block copolymer satisfies certain conditions such asa specific weight ratio of the vinyl aromatic hydrocarbon to theconjugated diene in the block copolymer, a specific molecular weight ofthe block copolymer, a specific storage modulus, a specific blockproportion of the vinyl aromatic hydrocarbon polymer, and a specificproportion of chains consisting of repeating units of the vinyl aromatichydrocarbon.

A process for making an extruded oriented sheet is well known in the artand is discussed further herein below. It is also known to those skilledin the art that the control of the film or sheet thickness, thetemperature of the film, and the draw ratios are important parametersthat define the film properties. Generally, materials having high meltstrength and retaining their orientation are considered better film orsheet forming materials than those having lower melt strength and notretaining their orientations.

It is also known in the art, that generally, polystyrene materials thatcontain branched structures possess higher melt strengths and havebetter processing characteristics than polystyrene materials thatpossess linear polymeric structures.

The above U.S. Pat. No. 5,830,924 assigned to The Dow Chemical Companydiscloses an example of a polystyrene for an extruded closed cell foamin which from 50 to 100 weight % of the polystyrene is branched.

A further example of a polystyrene material containing branchedstructures well suited for the preparation of blow molded articles,films, extruded foam, refrigerator liners, thermoformed articles andinjection molded articles is disclosed in U.S. Pat. No. 6,093,781issuing on Jul. 25, 2000 to Demirors, etal and assigned to The DowChemical Company. This patent also teaches away from the subject matterof the present invention that requires a different type of polymer,which, in turn requires a different type and weight % initiator, and alower weight % of a branched vinyl aromatic polymer.

The present invention seeks to provide a novel process for extrusionfoaming of styrenic polymers in which the styrenic polymer comprisesless than 50 weight % of branched styrenic polymer.

The present invention also seeks to provide for an oriented polystyrenearticle of styrenic polymers in which the styrenic polymer comprisesabout 50 weight % or less of branched styrenic polymer whereby thepolymer is prepared by solution or bulk polymerization in the presenceof from 0.01 to 0.1 weight % of a tetra functional peroxide initiator.

SUMMARY OF THE INVENTION

The present invention provides a closed cell foam comprising from C₈₋₁₂vinyl aromatic polymer comprising:

i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromatic monomers;and

ii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylic acid andacrylonitrile and methacrylonitrile;

which polymer may be grafted onto or occluded within from 0 to 12 weight% of one or more rubbery polymers selected from the group consisting of:

iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

iv) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile, saidvinyl aromatic polymer comprising 10 to 45 weight % of a star branchedpolymer and having a VICAT softening temperature not less than 100° C.

In a further embodiment the present invention provides a process forpreparing the above closed cell foam comprising injection into a moltenmass of C₈₋₁₂ vinyl aromatic polymer comprising:

i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromatic monomers;and

ii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylic acid andacrylonitrile and methacrylonitrile;

which polymers are grafted onto from 0 to 12 weight % of one or morerubbery polymers selected from the group consisting of:

iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

iv) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile, saidpolymer comprising 10 to 45 weight % of a star branched polymer andhaving a VICAT softening temperature not less than 100° C.; at atemperature from 140 to 235° C. and a pressure from 1500 to 3500 psifrom 2 to 15 weight % of one or more blowing agents selected from thegroup consisting of C₄₋₆ alkanes, CFCs, HCFCs, HFCs, CO₂ and N₂ andmaintaining said C₈₋₁₂ vinyl aromatic polymer in a molten state andthoroughly mixing said blowing agent with said polymer and extrudingsaid mixture of blowing agent and polymer.

The present invention also provides a process for polymerizing a vinylaromatic monomer comprising from 5 to 45 weight % of star branched vinylaromatic polymer, comprising feeding a mixture comprising:

i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromatic monomers;and

ii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylic acid andacrylonitrile and methacrylonitrile;

which polymer may be grafted onto or occluded within from 0 to 12 weight% of one or more rubbery polymers selected from the group consisting of:

iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

iv) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile, andfrom 0.01 to 0.1 weight % of a tetrafunctional peroxide initiator of theformula:

wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals; and R is a neopentyl group, in the absence of a cross linkingagent to a series of two or more continuous stirred tank reactors, toprovide a relatively low temperature initial reaction zone at atemperature from 100 to 130° C. and a relatively higher temperaturesubsequent reaction zone at a temperature from 130 to 160° C. andmaintaining a ratio of residence time in said relatively lowertemperature reaction zone to said relatively higher temperature reactionzone from 1:1 to 3:1 and recovering the resulting polymer, preferably,through devolatilization of unreacted monomers.

The present invention also provides a polymer composition comprisingC₈₋₁₂ vinyl aromatic polymer prepared by solution or bulk polymerizationin the presence of from 0.01 to 0.1 weight % of a tetra functionalperoxide initiator of the formula:

wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals and R is a neopentyl group, in the absence of a cross linkingagent. comprising:

i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromatic monomers;and

ii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylic acid andacrylonitrile and methacrylonitrile;

which polymer may be grafted onto or occluded within from 0 to 12 weight% of one or more rubbery polymers selected from the group consisting of:

iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

iv) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile, saidvinyl aromatic polymer comprising about 10 to about 50 weight % of astar branched polymer. The vinyl aromatic polymer preferably has a VICATsoftening temperature not less than 100° C.

The present invention also provides an oriented polystyrene article,e.g. film or sheet comprising from C₈₋₁₂ vinyl aromatic polymer preparedby solution or bulk polymerization in the presence of from 0.01 to 0.1weight % of a tetra functional peroxide initiator of the formula:

wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals and R is a neopentyl group, in the absence of a cross linkingagent, comprising:

i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromatic monomers;and

ii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylic acid andacrylonitrile and methacrylonitrile;

which polymer may be grafted onto or occluded within from 0 to 12 weight% of one or more rubbery polymers selected from the group consisting of:

iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

iv) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile] saidvinyl aromatic polymer comprising 10 to 45 weight % of a star branchedpolymer. The vinyl aromatic polymer preferably has a VICAT softeningtemperature of not less than 100° C.

In a further embodiment the present invention provides a process forpreparing the above oriented polystyrene article comprising injectioninto a molten mass of C₈₋₁₂ vinyl aromatic polymer prepared by solutionor bulk polymerization in the presence of from 0.01 to 0.1 weight % of atetra functional peroxide initiator of the formula:

wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals and R is a neopentyl group, in the absence of a cross linkingagent. comprising:

i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromatic monomers;and

ii) from 0 to 40 weight % of one or more monomers selected from thegroup consisting of C₁₋₄ alkyl esters of acrylic or methacrylic acid andacrylonitrile and methacrylonitrile;

which polymers are grafted onto from 0 to 12 weight % of one or morerubbery polymers selected from the group consisting of:

iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

iv) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile, saidvinyl aromatic polymer comprising about 10 to about 50 weight % of astar branched polymer and a VICAT softening temperature not less than100° C.;

maintaining said C₈₋₁₂ vinyl aromatic polymer in molten state andthoroughly mixing said polymer and extruding said polymer.

It is therefore an objective of the present invention to provide apolymer composition comprising a star branched polymer using atetra-functional peroxide initiator for use in extrusion foams withimproved melt strength compared to a polymer composition comprising alinear polymer.

It is a further objective of the present invention to provide a novelpolymer composition comprising a star branched polymer using atetra-functional peroxide initiator and its use in oriented articleswith improved melt strength and/or tensile properties compared to apolymer composition comprising a linear polymer.

BEST MODE

As used in this specification “star branched” polymer means havingmultiple, preferably at least 3, most preferably 4, branches eminatingfrom a common node.

Extrusion Foams:

The styrenic polymers of the present invention may be co- orhomopolymers of C₈₋₁₂ vinyl aromatic monomers. Some vinyl aromaticmonomers may be selected from the group consisting of styrene, alphamethyl styrene and para methyl styrene. Preferably the vinyl aromaticmonomer is styrene.

The styrenic polymer may be a copolymer comprising from 60 to 100 weight% of one or more C₈₋₁₂ vinyl aromatic monomers; and from 0 to 40 weight% of one or more monomers selected from the group consisting of C₁₋₄alkyl esters of acrylic or methacrylic acid and acrylonitrile andmethacrylonitrile. Suitable esters of acrylic and methacrylic acidinclude methyl acrylate, ethyl acyrlate, butyl acrylate, methylmethacrylate, ethyl methacrylate, butyl methacrylate, maleic anhydride,and fumaronitrile.

In a further embodiment of the present invention, the polymer for bothextrusion foams and oriented articles may be rubber modified. That is,the polymer may be grafted onto or occluded within from 0 to 12 weight %of one or more rubbery polymers selected from the group consisting of:

i) co- and homopolymers of C₄₋₅ conjugated diolefins; and

ii) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile.

The rubbery polymer may be selected from a number of types of polymers.The rubbery polymer may comprise from 40 to 60, preferably from 40 to 50weight % of one or more C₈₋₁₂ vinyl aromatic monomers which areunsubstituted or substituted by a C₁₋₄ alkyl radical and from 60 to 40,preferably from 60 to 50 weight % of one or more monomers selected fromthe group consisting of C₄₋₅ conjugated diolefins. Such polymers areknown as the styrene butadiene rubbers (SBR). The rubber may be preparedby a number of methods, preferably by emulsion polymerization. Thisprocess is well known to those skilled in the art and described forexample in Rubber Technology, Second Edition, edited by Maurice Morton,Robert E. Krieger Publishing Company Malabar, Florida, 1973, reprint1981—sponsored by the Rubber Division of the American Chemical Society.

The rubbery polymer may be a nitrile rubber comprising from 15 to 40weight % of one or more monomers selected from the group consisting ofacrylonitrile and methacrylonitrile, preferably acrylonitrile, and from85 to 60 weight % of one or more C₄₋₆ conjugated diolefins. The polymersmay be prepared by a number of methods, preferably by emulsionpolymerization or anionic, i.e. K-resin or Kraton process. Theseprocesses are well known to those skilled in the art and the former isdescribed for example in the aforementioned reference.

The rubber may be a co- or homopolymer of one or more C₄₋₆ conjugateddiolefins such as butadiene (1,3-butadiene) or isoprene, preferablybutadiene. The polybutadiene may have a molecular weight (Mw) from about260,000 to 300,000, preferably from about 270,000 to 280,000.Polybutadiene has a steric configuration. The polymer may have a cisconfiguration ranging from about 50% up to 99%. Some commerciallypolymers have a cis content of about 55% such as TAKTENE® 550 (trademarkof Bayer AG) or DIENE® 55 (trademark of Firestone). Some commerciallyavailable butadiene has a cis configuration from about 60 to 80% such asFirestone's DIENE® 570. Some high cis-butadiene rubbers may have a cisconfiguration of 95% or greater, preferably greater than 98% (TAKTENE®1202).

If present, preferably the rubber is present in an amount from about 3to 10% weight based on the total weight of the composition fed to thereactor (i.e. monomers and rubber). Polybutadiene is a particularlyuseful rubber.

The process for making HIPS (high impact polystyrene) is well known tothose skilled in the art. The rubber is “dissolved” in the styrenemonomer (actually the rubber is infinitely swollen with the monomer).The resulting “solution” is fed to a reactor and polymerized typicallyunder shear. When the degree of polymerization is about equal to theweight % of rubber in the system it inverts (e.g. the styrene/styrenepolymer phase becomes continuous and the rubber phase becomesdiscontinuous. After phase inversion the polymer is finished in a manneressentially similar to that for finishing polystyrene.

The polymer is prepared using conventional bulk, solution, or suspensionpolymerization techniques. However, there is added to the first reactor(i.e. the lower temperature reactor) from about 0.01 to 0.1 weight % (100 to 1000 ppm) of a tetrafunctional peroxide initiator of the formula:

wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals and R is a neopentyl group. The reaction is conducted in theabsence of a cross linking agent. Preferably the tetrafunctionalperoxide is present in the feed to the first reactor (i.e. the lowertemperature reactor) in an amount from about 200 to 400 ppm (0.02 to0.04 weight %), most preferably from 250 to 350 ppm (0.025 to 0.035weight %).

Suitable tetrafunctional peroxide initiators include initiators selectedfrom the group consisting of tetrakis-(t-amylperoxycarbonyloxymethyl)methane, tetrakis-(t-butylperoxycarbonyloxymethyl) methane,1,2,3,4-tetrakis (t-amylperoxycarbonyloxy) butane and the tetrakis(t-C₄₋₆ alkyl monoperoxycarbonates). A particularly useful initiator isthe compound of the above formula wherein R is a nenopentyl group and R¹is a tertiary amyl or tertiary butyl radical.

Typically in a bulk or solution process the monomer mixture andoptionally rubber is polymerized in at least two continuous stirred tankreactors. The first reaction temperature is kept at a relatively lowtemperature from about 100 to 130° C., preferably from 120 to 130° C.and then at a relatively higher temperature from about 130 to 160° C.,preferably from about 135 to 145° C. In the polymerization process thereare competing initiation reactions. The initiation may be thermalwithout the use of any additional initiator or may be initiated by theperoxy carbonate initiator. The residence time in each temperature zoneis controlled so that the amount of polymerization initiated thermally(which results in a linear polymer) and by the peroxy carbonateinitiator (in which about half of the resulting polymer is branched) iscontrolled so that not more than 50 weight % of the resulting polymer isbranched. For example if the reaction is controlled so that the ratio ofresidence time at the lower temperature to time at higher temperature isfrom 1:1 to 3:1, preferably from 1.5:1 to 2.5:1, most preferably about2:1 (i.e. 1.8:1 to 2.2:1). The weight ratio of linear to star branchedpolymer is controlled greater than 1:1 (e.g. greater than 50:50).Preferably, the vinyl aromatic polymer or styrenic polymer will comprisefrom about 10 to about 50, preferably from about 10 to about 50 or fromabout 15 to about 40 weight %, and most preferably from about 15 toabout 30 weight % of a star branched polymer.

In a suspension process the monomers, optionally including dissolvedrubber, may be either first partially polymerized in a continuouslystirred tank system. The partially polymerized monomer mixture hasstabilizers or suspending agents added to it to help suspend it in theaqueous phase as an oil-in-water suspension. Typically the stabilizer orsuspending agent is added in an amount from 0.1 to 2.0 weight %,preferably from 0.5 to 1.0 weight %.

Useful stabilizers, soaps, or suspending agents are well known to thoseskilled in the art. Useful stabilizers or suspending agents includepolyvinyl alcohol, gelatin, polyethylene glycol, hydroxyethyl cellulose,carboxymethyl cellulose, polyvinyl pyrrolidone, polyacrylamides, saltsof poly (meth) acrylic acid, salts of phosphonic acids, salts ofphosphoric acid and salts of complexing agents such as ethylene diaminetetraacetic acid (EDTA). Useful soaps include sodium N-dodecyl benzenesulfonate.

Generally the salts are ammonium, alkali and alkaline earth metal saltsof the foregoing stabilizers or suspending agents. For exampletricalcium phosphate is a suitable suspending agent.

The tetra functional initiator may be added to the monomer mixture priorto polymerization in the bulk or mass reactor or just prior tosuspension batch polymerization in the suspension batch reactor. Thesuspension batch reactor is generally operated at lower temperaturesthan the bulk reactor, i.e. typically 70 to 95° C. However, thesuspension batch reaction is finished at higher temperatures from about120 to 150° C., typically from about 125 to 135° C.

The resulting polymer has a number of unique properties that make itsuitable for extrusion foaming. The polymer has a VICAT softeningtemperature (as measured by DIN 53460 is equivalent to ISO 306 isequivalent to ASTM D 1525-96) of greater than 100° C., preferably from105° C. to 115° C. The polymer has mean melt strength at 210° C. of notless than 12.5 cN.

The melt strength and the stretch ratio test are determined using aRosand® C.apillary Rheometer. The mean melt strength is determined byextrusion of a melt at 210° C. of the polymer through a circular 2-mmdiameter flat die, where the length to diameter (L/D) of the die is20:1. The strand is extruded at a constant shear rate of 20 sec⁻¹. Thestrand is attached to a haul off unit that increases in speed with time.The strand is attached to a digital balance scale to measure the forceof draw on the polymer. As the speed of the haul off unit increases, thedraw force increases. As a result the strand breaks. The draw forceimmediately prior to break is defined as the melt strength. The stretchratio is defined as the ratio of the velocity of draw to the extrusionvelocity at the die exit. The test is repeated at least three times todetermine an average value.

The polymer may have a melt flow at condition G (200° C./5 kg) load ofless than 5 grams/10 minutes, preferably less than 3 grams/10 minutes,most preferably of less than 2 grams/10 minutes. Additionally, thepolymer has a Mz which exceeds typical high heat crystal polystyreneresins by at least 40,000, preferably by greater than 60,000.

The polymer may be foamed using conventional extrusion foamingequipment. The extruder may be a back to back type or it may be amultizoned extruder having at least a first or primary zone to melt thepolymer and inject blowing agent and a second extruder or zone.

In the primary extruder or zone the polymer melt is maintained attemperatures from about 425° F. to 450° F. (about 218 to 232° C.). Oncethe polymer is melted, blowing agent is injected into the melt at theend of the primary extruder or zone. In the primary extruder or zonethere will be a high shear zone to promote thorough mixing of theblowing agent with the polymer melt. Such a zone may comprise a numberof pin mixers.

The polymer melt containing dissolved or dispersed blowing agent is thenfed from the primary extruder to the secondary extruder or passes from aprimary zone to a secondary zone within the extruder maintained at amelt temperature of 269° F. to 290° F. (about 132° C. to 143° C.). Inthe secondary extruder or zone the polymer melt and entrained blowingagent passes through the extruder barrel by the action of an auger screwhaving deep flights and exerting low shear upon the polymer melt. Thepolymer melt is cooled by means of cooling fluid, typically oil whichcirculates around the barrel of the extruder. Generally the melt iscooled to a temperature of from about 250° F. to about 290° F. (about121° C. to 143° C.).

The blowing agent may be selected from the group consisting of C₄₋₆alkanes, CFCs, HFCs, HCFCs, CO₂, N₂, air and mixtures thereof. Theblowing agent may be CO₂ per se or N₂ per se. The blowing agent maycomprise from 20 to 95 weight % of a blowing agent selected from thegroup consisting of one or more C₄₋₆ alkanes (as described below) andfrom 80 to 5 weight % of CFCs, HFCs and HCFC's (as described below).Suitable C₄₋₆ alkanes include butane, pentane and mixtures thereof.

The blowing agent may comprise from 30 to 95, preferably from 70 to 95,most preferably from 80 to 90 weight % of CO₂ and from 70 to 5,preferably from 30 to 5, most preferably from 20 to 10 weight % of oneor more compounds selected from the group consisting of C₁₋₂ halogenatedalkanes and C₄₋₆ alkanes. Suitable C₁₋₂ halogenated alkanes include thechloroflurocarbons (CFCs); hydrofluorocarbons (HFCs) and thehydrochlorofluorocarbons (HCFCS) such as trichlorofluoromethane(CFC-11); dichlorodifluoromethane (CFC-12); trichlorotrifluoroethane(CFC-113); dichlorotetrafluoroethane (CFC-114); dichlorofluoromethane(CFC-21); chlorodifluoromethane (HCFC-22); difluoromethane (HFC-32);2-chloro-1,1,1,2-tetrafluoroethane (HCFC-124); pentafluoroethane(HFC-125); 1,1,1,2-tetrafluoroethane (HCFC-124);1,1-dichloro-1-fluoroethane (HCFC-141b); 1-chloro-11-difluoroethane(HCFC-142b); trifluoroethane (HFC-143a); 1,1-difluoroethane (HFC-152a);tetrafluoroethane (HFC-134a); and dichloromethane. However, due toenvironmental concerns it is preferred to use alkanes such as C₄₋₆alkanes which have not been halogenated such as butane, pentane,isopentane and hexane. The blowing agent system may be used in amountsfrom 2 to 15, preferably from 2 to 10, most preferably from about 3 to 8weight % based on the weight of the polymer.

The pressure within the extruder should be sufficient to keep theblowing agent in the polymer melt. Typically, the pressures in the meltafter the blowing system has been injected will be from about 1500 to3500 psi, preferably from about 2000 to about 2500 for CO₂. The CO₂ andthe other blowing agent may be injected separately into the melt. Ifthis is done, preferably the alkane and/or halogenated alkane will beinjected upstream of the CO₂ as these types of blowing agents have aplasticizing effect on the polymer melt that may help the CO₂ go intothe melt. The alkane blowing agent and the CO₂ may also be mixed priorto injection into the extruder as is disclosed in U.S. Pat. No.4,424,287 that issued on Jan. 3, 1984 and that is assigned to Mobil OilCorporation.

To improve the cell size and/or distribution throughout the polymersmall amounts of a nucleating agent may be incorporated into the polymerblend or solution. These agents may be physical agents such as talc orthey may be agents that release small amounts of CO₂ such as citric acidand alkali or alkaline earth metal salts thereof and alkali or alkalineearth metal carbonates or bicarbonates. Such agents may be used inamounts from about 500 to 5,000 ppm, typically from about 500 to 2,500ppm based on the polymer melt or blend.

The polymer melt or blend may also contain the conventional additivessuch as heat and light stabilizers (e.g. hindered phenols and phosphiteor phosphonite stabilizers) typically in amounts of less than about 2weight % based on the polymer blend or solution; typically from 200 to2,000 parts per million (ppm).

The foam is generally extruded at atmospheric pressure and as a resultof the pressure decrease, the melt foams. The foam is cooled to ambienttemperature typically below about 25° C., which is below the glasstransition temperature of the polymer and the foam is stabilized. One ofthe advantages of the present invention is that the foamed polymer melthas better melt strength than the foamed polymer melts of the prior artand there is less foam collapse and ruptured cells (open cellstructure).

The foam may be extruded onto rollers as a relatively thick slabtypically from about 1 to 3 inches thick. The foam density may vary from2 to 15 lbs/ft³ (from about 0.03 to 0.24 grams/cm³). The slab is cutinto appropriate lengths (8 feet) and is generally used in theconstruction industry. Thinner foams, typically from about {fraction(1/16)} to about ¼ inches (62 to 250 mils) thick may be extruded asslabs or as thin walled tubes which are expanded and oriented over anexpanding tubular mandrel to produce a foam tube which is slit toproduce sheet. These relatively thin sheets are aged, typically 3 or 4days and then may be thermoformed into items, such as coffee cups, meattrays or “clam shells”.

The present invention will now be illustrated by the followingnon-limiting examples in which, unless otherwise indicated parts meansparts by weight (grams) and percent means weight percent.

Examples 1 through 4 pertain to extrusion foam.

EXAMPLE 1 Polymer Preparation

Styrene monomer and 0.028 weight % of a tetra t-alkylperoxy carbonatesold by Ato Fina under the trade mark JWEB50 were first fed into acontinuously stirred tank reactor maintained at 120° C. The residencetime in the first reactor was about 2.5 hours. The partially polymerizedmixture from the first reactor was then fed to a second continuouslystirred tank reactor maintained at 140° C. The residence time in thesecond reactor was about 1 hour. The resulting polymer was thendevolatilized in a falling strand devolatilizer and recovered andpelletized.

The reaction conditions were such that about 64% of the polymer wasthermally initiated and linear. About 36% of the polymer was initiatedby the peroxide and about half of the resulting polymer was starbranched. The polymer had a Mz from 40,000 to 75,000 greater thanconventional high heat crystal.

EXAMPLE 2

The procedure of Example 1 was repeated except that the amount ofinitiator was 0.045 weight %.

EXAMPLE 3

The procedure of Example 1 was repeated except that zinc stearate wasalso included in the polymer in an amount of about 0.1 weight %.

Physical Properties

The physical properties of the resins prepared in Examples 1, 2 and 3were compared to commercially available linear polystyrene resins A, B,and C used in extruded foam applications. The results are set forth inTable 1.

EXAMPLE 4

The above samples together with the reference samples were extrusionfoams using pentane as the blowing agent. The average cell diameter ofthe foam was measured. The results are set out in Table 2.

The foams extruded well and the cell data suggests that the foamstability is good. The resulting foams have good toughness.

TABLE 1 Polystyrene Sample Identification Example 1 Example 2 Example 3Resin A Resin B Resin C Initiator: polyether tetrakis 280 ppm 450 ppm280 ppm Initiator Initiator Initiator + Zn (t-butylperoxy carbonate) Mw351,000 345,000 342,000 306,000 309,000 310,000 Mn 132,000 113,000141,000 77,000 102,000 130,000 Mz 638,000 659,000 606,000 535,000551,000 550,000 Polydispersity (Mw/Mn) 2.66 3.05 2.42 3.97 3.03 2.38Mean Melt Strength (cN) @ 190° C. 38.21 36.95 37.44 31.07 30.42 34.7Mean Stretch Ratio (%) @ 190° C. 91.8 81 79.9 84.3 99.4 91.8 Mean PeakMelt Strength @ 190° C. 45.25 45.73 43.7 38.74 34.75 39.73 Mean MeltStrength (cN) @ 210° C. 14.11 14 14.56 10.25 11.02 11.95 Mean StretchRatio (%) @ 210° C. 279.6 230.3 236.5 428.6 399.4 326 Mean Peak MeltStrength @ 210° C. 17.43 17.02 17.73 12.22 12.87 14.43 Notched Izod(ft-lb/in) 0.36 0.32 0.34 0.33 0.32 0.22 Melt Flow Condition “G”(grams/10 1.35 1.74 1.4 1.98 2.07 1.42 min) VICAT (° C.) 108.4 109 108.9108.2 108.6 109.9

TABLE 2 Polystyrene Sample Identification Example 1 Example 2 Example 3Resin A Resin B Resin C Initiator: polyether tetrakis 280 ppm 450 ppm280 ppm Initiator Initiator Initiator + Zn (t-butylperoxy carbonate)Isopentane fed to foam process (wt %) 5 5 5 5 5 5 Test Results on 2SType Foamed Meat Trays Molded From Polystyrene Samples Mean Load at MaxLoad (lbs) 2.44 2.56 2.84 3.2 2.88 2.96 Mean Displacement at Max Load(in.) 2.16 1.92 1.46 1.71 2.13 2.28 Mean Load at 1.5″ Deflection lbs 2.22.44 2.82 3.16 2.67 2.62 Mean Slope (lbs/in) 3.02 3.86 3.58 3.74 3.613.47 Mean Part Weight (grams) 4.521 4.595 4.59 4.96 4.758 4.93 MeanSidewall Thickness (inches) 0.091 0.087 0.105 0.102 0.103 0.098 MeanFoam Density (lbs/ft³) 3.2 3.56 2.92 3.07 3.02 3.194 Mean Orientation MD(%) 55.93 57.48 56.07 54.4 56.68 54.49 Mean Orientation TD (%) 57.1557.2 56.55 54.73 56.47 52.29 Number of Cells Across Sheet Thickness (TD)21 20 38 22 31 23 Average Cell Diameter(mm) 0.1101 0.1105 0.0702 0.11780.0844 0.1082 Number of Parts With Sidewall Cracks 0 1 0 3 0 2 CellStructure coarse fine coarse coarse Cell Shape slight slight sphericalspherical spherical spherical elongation elongation Corner InversionTest on Trays - Failure 0 3 0 2 0 6 Rate/20

Oriented Articles

The styrenic polymer composition of the invention may be used in thepreparation of oriented articles, e.g. blown film and extruded sheetthat are oriented uniaxially or biaxially. In this instance, the aboveteachings and/or components of the polymer composition for the extrusionfoams will apply for the polymer composition used in oriented articlesof the invention except that blowing agents and nucleating agents arenot required in the polymer composition for oriented blown film andoriented extruded sheet.

For the manufacture of an oriented extruded sheet, the polymer resingranules are fed into an extruder where the resin is heated to a moltenstate having a polymer melt temperature ranging between 200° C. and 250°C., preferably about 230° C., and then extruded through a sheet T- dieand onto a roll stack. The roll stack is operated such that thetemperature of the material is above its Tg (glass transitiontemperature). The roll stack imparts a high degree of orientation(>100%) in the machine direction (MD). As the sheet comes off the rollstack the sides of the sheet are engaged by a series of clamps that areattached to a continuous chain. The clamps pull the sheet through a“tenter frame” which is a long oven. The oven has several heating zonesin which the temperature of the material is maintained above its Tg. Asthe sheet is carried through the tenter oven, the continuous chain ofclamps begins to diverge thereby imparting a high degree of orientation(>100%) in the transverse direction (TD). In order for the temperatureof the material to remain above its Tg, the temperature in each heatingzone is set above the Tg of the polymer. For polystyrene having a Tg ofabout 105° C., the temperature in each heating zone will typically beset at about 118° C.

Depending on the final application of the sheet, the amount oforientation imparted in the machine direction (MD) and in the transversedirection (TD) will vary. For many applications, the stretch ratios forMD and TD are usually the same or balanced, e.g. MD=TD=2:1 draw ratio.That is, a cookie tray will typically require a 2:1 draw ratio in boththe MD and TD, while an envelope window will require a draw ratio of 7:1in the MD and TD. However, if a hinge is associated with the part, thesheet may require an unbalanced biaxial orientation in the MD and TDorientation, e.g. MD 2:1 draw ratio while TD=1.3:1 draw ratio.Typically, an oriented polystyrene sheet having a 2:1 draw ratio in boththe MD and TD will have shrink tension of about 100 pounds per squareinch (psi) as measured according to ASTM D 2838.

This pulling or orientation in the machine direction and transversedirection may be done simultaneously or sequentially. When polystyreneis oriented using the process described above, generally it is donesequentially where the sheet is first pulled in the machine directionand then pulled in the transverse direction.

Other additives can be added to the polymer composition for orientedarticles. Further examples of suitable additives are softening agents;plasticizers, such as cumarone-indene resin, a terpene resin, and oilsin an amount of about 2 parts by weight or less based on 100 parts byweight of the polymer; pigments; anti-blocking agents; slip agents;lubricants; coloring agents; antioxidants; ultraviolet light absorbers;fillers; anti-static agents; impact modifiers. Pigment can be white orany other color. The white pigment can be produced by the presence oftitanium oxide, zinc oxide, magnesium oxide, cadmium oxide, zincchloride, calcium carbonate, magnesium carbonate, etc., or anycombination thereof in the amount of 0.1 to 20% in weight, depending onthe white pigment to be used. The colored pigment can be produced bycarbon black, phtalocianine blue, Congo red, titanium yellow or anyother coloring agent known in the printing industry.

Examples of anti-blocking agents, slip agents or lubricants are siliconeoils, liquid paraffin, synthetic paraffin, mineral oils, petrolatum,petroleum wax, polyethylene wax, hydrogenated polybutene, higher fattyacids and the metal salts thereof, linear fatty alcohols, glycerine,sorbitol, propylene glycol, fatty acid esters of monohydroxy orpolyhydroxy alcohols, phthalates, hydrogenated castor oil, beeswax,acetylated monoglyceride, hydrogenated sperm oil, ethylenebis fatty acidesters, and higher fatty amides. The organic anti-blocking agents can beadded in amounts that will fluctuate from 0.1 to 2% in weight.

Examples of anti-static agents are glycerine fatty acid, esters,sorbitan fatty acid esters, propylene glycol fatty acid esters, stearylcitrate, pentaerythritol fatty acid esters, polyglycerine fatty acidesters, and polyoxethylene glycerine fatty acid esters. An anti-staticagent may range from 0.01 to 2% in weight. Lubricants may range from 0.1to 2% in weight. A flame retardant will range from 0.01 to 2% in weight;ultra-violet light absorbers will range from 0.1 to 1%; and antioxidantswill range from 0.1 to 1% in weight. The above compositions areexpressed as percent of the total weight of the polymer blend.

Fillers, such as talc, silica, alumina, calcium carbonate, bariumsulfate, metallic powder, glass spheres, and fiberglass, can beincorporated into the polymer composition in order to reduce cost or toadd desired properties to the film or sheet. The amount of fillerpreferably will be less than 10% of the total weight of the polymercomposition as long as this amount does not alter the shrinkingproperties of the film or sheet when temperature is applied thereto.

The polymer composition for the oriented article of the invention,particularly extruded polystyrene sheet, may comprise impact modifiers.Examples of impact modifiers include high impact polystyrene (HIPS),styrene/butadiene block copolymers, styrene/ethylene/butene/styrene,block copolymers, styrenelethylene copolymers. The amount of impactmodifier used is typically in the range of 0.5 to 25% of the totalweight of polymer.

The oriented film or sheet of the invention can be used in any of thewell-known food packaging processes, such as in the preparation ofyogurt cups, cake domes, cookie trays, envelope windows, CD jewel boxshrink film packaging, trays of all sizes and shapes for general foodpackaging and vending cups. The food packaging process typicallyinvolves the polymer film or sheet having a thickness of a fewmillimeters (typically between 0.2 mm and 0.6 mm). The extruded orientedfilm or sheet is fed to one or more heating ovens where its temperatureis increased above the glass-transition temperature of resin.

Once the desired temperature is reached, the sheet or film is formedinto the desired shape by known processes such as plug assistedthermoforming where a plug pushes the sheet or film into a mold of thedesired shape. Air pressure and/or vacuum can also be employed to moldthe desired shape. During the orientation processing of the film orsheet, the molecules are aligned in both the MD and TD directions.Molecular alignment has long been known to significantly increase theoverall toughness of the resin. Thus, when a formed article or part ismade from the extruded oriented film or sheet of the invention, theformed article or part retains the “toughness” characteristic impartedto the film or sheet during the orientation process.

The polymer composition of the oriented polystyrene article preferablyhas a VICAT softening temperature (as measured by DIN 53460 isequivalent to ISO 306 is equivalent to ASTM D 1525-96) of greater than100° C., preferably from 100° C. to 115° C., and more preferably from105° C. to 115° C. Also, the polymer composition has a mean meltstrength at 210° C. of not less than 12.5 cN, and a melt flow atcondition G of less than 2.5 grams/10 minutes.

Preferably, the polymer composition of the oriented article of theinvention is polystyrene. Preferably, the oriented article of theinvention has a tensile strength ranging from about 8,000 pounds persquare inch (psi) to about 12,000 pounds per square inch in atemperature range of about 20° C. to about 30° C. and a tensile strengthranging from about 9,000 pounds per square inch to about 15,000 poundsper square inch in a temperature range from about −20° C. to about −40°C.

EXAMPLE 5

Example 5 pertains to an oriented polystyrene article. The star branchedpolystyrene resin (0.0280 weight % initiator) used in the polymercomposition of the invention is the same as Example 1 of Table 1 herein.The oriented polystyrene article made from Example 1 of Table 1 isidentified herein as “Sample I”. For comparative purposes, acommercially available high molecular weight linear polystyrene resinwas used for all sample preparations and testing. This linear resin isidentified herein as “Resin D”. Resin D comprises greater than 99.5%polystyrene with about 0.1% to 0.3% mineral oil. Typical chemical andphysical properties for Resin D prior to being subjected to anorientation process are shown in Table 3.

TABLE 3 Resin D - Typical Properties (un-oriented) Tensile % Tensile @Modulus Elongation MFI @ Mw × 10⁻³ Mn × 10⁻³ Mz × 10⁻³ Yield (psi) ×10⁻⁵ on 200° C. 345 133 626 7,580 503 2.53 1.6

Sample Preparation:

Twenty-five compression molded plaques were made from each polymercomposition of Sample I and Resin D by using a Pasadena Hydraulic PressModel #SQ 33-C-X-MS-X 24. These plaques were 4.5″ wide, 4.5″ long and100 mil thick. The conditions used for making the plaques are listed inTable 4.

TABLE 4 Sample Preheat Preheat Mold Compression Cooling Cooling WeightTime Pressure Temp Compression Pressure Time Temp. (grams) (min.) (psi.)(° F.) Time (min.) (psi) (min.) (° F.) 26 5 100 450 5 30,000 5 68

At least ten compression-molded plaques for both Sample I and Resin Dwere used as samples.

Orientation Process:

A lab scale film stretcher was used to simulate a commercially biaxialorientation process. This film stretcher, Model No BIX-702 manufacturedby Iwamoto of Japan, has two movable draw bars mounted on anelectrically driven jack screw and located at right angles to eachother. A stationary draw bar is located opposite to each movable drawbar. Each draw bar has pneumatically actuated clips for holding a samplein position upon operation of the stretcher.

The general operation of the film stretcher was as follows. A sample wasplaced in the middle of the draw bar arrangement and the cover waslowered over the sample. The sample was heated for three minutes tosoften the material so that the clamps could properly grasp thematerial. The clamps were activated to grasp the material. Heating wascontinued at 120° C. (preheat temperature) for 15 minutes (preheattime). Each sample was oriented using the sequential orientation processwhere the sample was stretched in the machine direction (MD) first andthen stretched in the transverse direction (TD). The final dimension ofeach sample was 12″×12″. The stretching may be done at any selectedstrain rate but for this Example 5, a strain rate of 540% per minute wasused. These parameters used in stretching the samples are shown in Table5.

TABLE 5 Strain Rate Presoak Initial Final mm/sec. Presoak Temperature %Total Sample Size Sample Size (%/min.) Time (min.) ° C. Strain 4.5″ ×4.5″ 12″ × 12″ 9 mm/sec. 15 120 200 540%/min. (Note: % Total Strain =Change in dimension compared to original dimension, i.e. (finaldimension - initial dimension)/initial dimension.)

Orientation Measurements:

Molecular orientation is generally measured by taking birefringencemeasurements on the uniaxially or biaxially oriented film or sheet.However, for this Example 5, an approximate value for the amount oforientation was obtained by measuring the shrinkage of the plaque afterit had been exposed to a temperature above its Tg. To do this, three4″×4″ samples were cut from the oriented plaques formed in the filmstretcher. Each 4″×4″ sample was marked with a pencil such that 9 markswere spaced 1″ apart. Each sample was lightly coated with talc andplaced on a ¼ inch spacer between two aluminum plates. The samples werethen placed in a circulating air oven set at a temperature of 163° C.for 30 minutes. The samples were removed from the oven, air cooled, andthe distance between each mark was measured. The linear shrinkage wasthen calculated for both the machine direction (MD) and the transversedirection (TD). The % linear orientation in the MD and the TD wascalculated as follows: % Linear Orientation=((Initial Length-FinalLength)/Final Length)×100%); machine direction being the direction inwhich the sample was pulled first and the transverse direction being thedirection in which the sample was pulled second. The results were takenas an average for the samples and are shown in Table 6.

TABLE 6 Polymer % Linear Shrinkage - MD % Linear Shrinkage - TD Sample I198.0 177.0 Resin D 216.8 175.6

From the data in Table 6, it can be seen that the samples for bothSample 1 and Resin D have about the same degree of orientation in boththe machine and transverse directions.

Tensile Yield:

Testing was done on the biaxially stretched samples according to ASTMD-638-99. These samples were the 12″×12″ samples produced in the aboveOrientation Process. These samples were tested for their tensileproperties at 23° C. (room temperature) and at −34° C. (refrigerationtemperature). This lower temperature testing was done because of thepotential end use application of the polymer composition of Sample I inpastry containers, etc. that are subjected to refrigeration, especiallyduring shipment of the products packaged in the containers made from thepolymer composition comprising Sample I. The tensile properties for thesamples containing the polymer composition of Sample I and Resin D arelisted in Table 7.

TABLE 7 Tensile Tensile Testing Stress @ Energy to Testing TemperatureYoung's Break % Strain Break (in Polymer Direction ° C. Modulus (psi) @Break lb/in)* Sample I MD  23 214  9,570 6.93 485.00 Resin D MD  23 230 9,040 5.93 381.32 Sample I MD −34 230 11,960 5.83 411.26 Resin D MD −34— 11,160 4.13 276.72 Sample I TD  23 223  9,700 7.27 541.00 Resin D TD 23 239  9,430 6.06 409.30 Sample I TD −34 221 12,460 6.34 475.0  ResinD TD −34 240 12,000 5.69 409.0  *Energy to Break = Normalized area undertensile stress-strain curve.

The data shown in Table 7 indicates that the samples containing thepolymer composition of Sample I have better tensile toughnesscharacteristics than that comprising the polymer composition of Resin Dunder both extreme temperatures. On an average, the values for the“Tensile Stress @ Break” are 5% higher for Sample I compared to that ofResin D while the values for the “Tensile Energy to Break” for Sample Ion an average are 32% higher compared to that for Resin D.

It has been illustrated that the star-branched nature of Sample I mayproduce a tougher oriented polystyrene product compared to the linearnature of Resin D.

What is claimed is:
 1. A process for producing an oriented polymer article comprising: (a) preparing a molten mass of C₈₋₁₂ vinyl aromatic polymer prepared by polymerization in the presence of from 0.01 to 0.1 weight % of a tetra functional peroxide initiator of the formula:

wherein R¹ is selected from the group consisting of C₄₋₆ t-alkyl radicals and R is a neopentyl group, in the absence of a cross linking agent, and comprised of: i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromatic monomers; and ii) from 0 to 40 weight % of one or more monomers selected from the group consisting of C₁₋₄ alkyl esters of acrylic or methacrylic acid and acrylonitrile and methacrylonitrile; which polymer may be grafted onto or occluded within from 0 to 12 weight % of one or more rubbery polymers selected from the group consisting of: iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and iv) copolymers comprising from 60 to 85 weight % of one or more C₄₋₅ conjugated diolefins and from 15 to 40 weight % of a monomer selected from the group consisting of acrylonitrile and methacrylonitrile, said vinyl aromatic polymer comprising about 10 to about 50 weight % of a star branched polymer, and while maintaining the temperature of said vinyl aromatic polymer above its Tg temperature, first forming said vinyl aromatic polymer into said polymer article and then imparting a high degree of orientation to said polymer article in the transverse and machine directions for said producing of said oriented polymer article.
 2. The process of claim 1 wherein in step a) said molten mass is prepared by heating said vinyl aromatic polymer to a temperature ranging between about 200° C. and about 250° C., and wherein said oriented polymer article of step b) is an extruded, oriented polystyrene sheet.
 3. The process of claim 1 wherein said vinyl aromatic polymer is prepared by solution or bulk polymerization.
 4. The process of claim 1 wherein said star branched vinyl aromatic polymer has a VICAT softening temperature not less than 100° C.
 5. The process of claim 1 wherein said star branched vinyl aromatic polymer is present in an amount from about 15 to about 50 weight % of the vinyl aromatic polymer.
 6. The process of claim 5 wherein the vinyl aromatic polymer has a mean melt strength at 210° C. of not less than 12.5 cN.
 7. The process of claim 6 wherein the vinyl aromatic polymer has a VICAT softening temperature from 105 to 115° C.
 8. The process of claim 1 wherein the tetrafunctional initiator is selected from the group consisting of tetrakis-(t-amylperoxycarbonyloxymethyl) methane, and tetrakis-(t-butylperoxycarbonyloxymethyl) methane.
 9. The process of claim 8 wherein the vinyl aromatic polymer has a melt flow at condition G of less than 2.5 grams/10 minutes.
 10. The process of claim 1 wherein the oriented polymer article has a tensile strength ranging from about 8,000 pounds per square inch to about 12,000 pounds per square inch in a temperature range of from about 20° C. to 30° C. and a tensile strength ranging from about 9,000 pounds per square inch to about 15,000 pounds per square inch at a temperature range of about −20° C. to about −40° C. 